"The rail traffic to Cuba, as with Mexico, was mainly in US owned cars.
However new cars built in the US for the Consolidated Railways of Cuba, National of Mexico or other foreign railroads using the AAR coupler, standard gauge and Westinghouse air brake, had their equipment roll on US rails from the factory to a shipping point."
All of them were painted and usually lettered, wearing Metric dimensional and weight data. For US operation, English Empire data would also be required (feet, inches, pounds and tons).
Note that Canadian freight cars carry both, as well as lettering in English and French.
So modeling a brand new car for NdeM or Cuba Consolidated and running it on a US theme model railroad would be a stretch but not incorrect.
Mainly because there would be more than one car of them in a train for delivery at the border, or a port for shipment abroad."
You are correct regarding the Cuban Railroads, but incorrect about Mexican roads. The various Mexican roads interchanged cars with several US roads which served the Southwest. Any ORER will list the interchange points.
A good number of Mexican railroads were members of The MCBA, The ARA and AAR and adhered to the Interchange Rules.